An OS 60 2-stroke will produce 7 - 8 lbs of static thrust. Your 91 2-stroke will produce 10 - 11 lbs. An OS91 surpass will produce about 10 lbs - I have one, that is what I get. I like about 1.5 or more thrust to weght for 3d. If I were in your situation I would get a Saito 91 or maybe a Magnum 4-stroke. My Surpass is a good engine but I like my Saito a little better and it is lighter.
In general 4-stokes run better in the half to 3/4 throttle range, smoother resopnse and quicker spool-up than comparable 2-stroke engines. I also have aYS 63 that is smooth in transition, light and powerful. I fly both 2-stroke and 4-stroke engines and don't fuss much with tuning either.